Denitration by Activated Carbon

Nitrogen oxides (NOx) are one of the major environmental pollutants that will cause problems such as acid rain, photochemical smog and ozone depletion. At present, the most widely used flue gas denitration method in coal-fired power plants is the selective catalytic reduction (SCR) method. However, the SCR method requires the addition of reducing substances such as NH3 during the reaction process, which is not applicable in the case of intermittent NOx emissions in catalyst plants with large concentration changes.

As an alternative, activated carbon denitration technology is used to meet the environmental protection requirements. The removal effect and process conditions of activated carbon as a catalyst and reactant for removing NOx are discussed as follows, along with a comparison of the reaction performance and conditions of activated carbon impregnated with 5% iron(III) oxide and unmodified activated carbon.

The Influence of temperature on the denitration performance of activated carbon

Based on the change curves between reaction time and NO concentration of different temperatures from 150 °C to 300 °C, when other conditions remain unchanged, the higher the reaction temperature, the better the denitration effect of the tested unmodified activated carbon. The denitration rates at 150 °C and 250 °C when reaching equilibrium are 23.2%, 36.6%, respectively. Whereas that at 300 °C is 80.3%.

As activated carbon has a strong adsorption effect on NO, O2, CO, CO2, it takes more time to reach equilibrium. When the gas begins to enter the reactor, the adsorption of activated carbon plays a leading role in the denitration rates. At equilibrium, the reaction between activated carbon and reaction gas takes the lead. And the reaction product concentration being stable indicates that the reaction rate is fast.

Denitration performance comparison of unmodified activated carbon and 5% iron(III) oxide impregnated activated carbon

The comparison test is set at 300 °C. The denitration rate of 5% iron(III) oxide impregnated activated carbon is able to reach 86%, but the conversion of NO has no change. It shows that iron(III) oxide has a catalytic effect on the reduction of NO by CO and can reduce the activation energy of the reaction. The adsorption and desorption equilibrium time required for 5% iron oxide activated carbon is greatly shortened, which may be because the specific surface area of the activated carbon decreases after the iron oxide is impregnated, and the adsorption capacity for NO decreases, so the time to reach equilibrium is shortened.

Influence of space velocity on denitration performance

Space velocity is the relation between volumetric flow rate and reactor volume in a chemical reactor. Based on the test results, when the space velocity of the iron(III) oxide impregnated activated carbon is reduced by one time, the NO concentration is reduced by more than one time, and the NO removal rate at equilibrium can reach 96%. It can be seen that reducing the space velocity is beneficial to the NO treatment, which indicates that increasing the residence time has a promoting effect on the denitration of activated carbon. But, if the space velocity is too low, the processing efficiency will be greatly affected, so the velocity needs to be controlled within a certain range.